1. Field of the Invention
The invention relates to unsaturated compounds containing carbamate or urea end groups and to their use in dual cure systems.
2. Description of the Background
The feature of dual cure systems is that they are curable by means of two independent curing mechanisms, e.g., radiation curing, moisture curing, oxygenated curing or thermal curing. There is a particular desire for systems which following the application of the coating composition may be precured by very short exposure to UV light to give a flexible film which is dust-dry. This film should then either be thermally postcurable or should continue to cure just on storage in air over the course of a few days until a hard film has formed which has the final desired service properties. This type of two-stage curing is of particular importance since it allows the processors of the coating systems to coat an article with a film in a first workstep and in a second step to process said film further, in particular to give the ready-coated and irradiated article a certain profile using pressure. At the time of their deformation in the second workstep, therefore, the films or sheets must have already undergone initial curing, so that they do not stick to the tools in the course of deformation; on the other hand, however, they must not already be so hard that they tear when stretched and deformed.
The phrase “dual cure” refers in accordance with the invention to a curing process which takes place by way of at least two mechanisms, namely radiation, moisture, oxidative and/or thermal curing.
Dual cure coatings which are photochemically polymerizable and possess a secondary curing mechanism, by way of alkoxysilane groups, for example, are known from U.S. Pat. No. 5,523,443; such structures may also contain carbamate linkages. Said document describes polymerizable systems which can be cured both by ultraviolet exposure and by way of a further mechanism. These systems consist of an alkoxysilyl-urethane(meth)acrylate and a (meth)acrylate or vinyl ether as reactive diluent.
Moreover, U.S. Pat. No. 5,075,384 discloses unsaturated carbamates obtained by reacting unsaturated isocyanates with the hydroxyl groups of styrene-allyl alcohol copolymers. The carbamate groups that have formed serve merely as linkers for the introduction of unsaturated groups. The systems described therein are curable by UV and dry oxidatively.
Further documents, examples being U.S. Pat. Nos. 4,138,299, 4,173,682, 4,415,604, and EP-A 549 116, describe resins which can be cured by means of radiation and atmospheric moisture. Both the oxidative drying and the air curing, however, are very slow processes which proceed in an uncontrolled fashion.
U.S. Pat. No. 4,138,299 describes a radiation- and moisture-curable coating system composed of 15-60% by weight of acrylate as reactive diluent and 40-85% by weight of an isocyanate-terminated prepolymer formed by reacting polyesterdiols and -triols with aliphatic diisocyanates, 5-15% of the available isocyanate groups being reacted with hydroxy acrylates.
U.S. Pat. No. 4,173,682 describes a method of coating a substrate with a radiation- and moisture-curable coating comprising an isocyanato-containing adduct of a (meth)acrylic acid hydroxy ester and a polyisocyanate, a polyfunctional hydroxy compound, and a photoinitiator. The components may not be combined until shortly before application in order to prevent unwanted hardening.
U.S. Pat. No. 4,415,604 describes a UV- and moisture-curable coating composition comprising a polyetherdiol or -triol containing isocyanate end groups, a (meth)acrylate as reactive diluent, and a polymerization initiator.
EP-A 549 116 describes a dual cure composition which comprises reactive diluents and an isocyanate adduct that carries free isocyanate groups and free, photopolymerizable, ethylenically unsaturated groups.
Unsaturated isocyanato acrylates of this kind which contain not only the photochemically curable acrylate groups but also free isocyanates are, however, not stable on storage or else must be stored in the absence of moisture, which is awkward.
EP-A 844 286 describes a powder coating system for powder coating materials that comprises not only a UV free-radical initiator but also an initiator which can be activated thermally. This text remarks that liquid systems based on thermal peroxide initiators only have relatively short storage lives.
Moreover, systems which cure by means of heat alone and which crosslink by way of carbamates or ureas are known—from U.S. Pat. No. 5,770,650 for example. U.S. Pat. No. 5,770,650 describes a curable composition containing carbamate or urea and ester or amide groups and a curing reagent. The possibility of a UV cure is not contemplated.
Also known are unsaturated isocyanato acrylates which in addition to the photochemically curable acrylate groups also contain free isocyanates. However, such binders lack storage stability.
More stable on storage are the radiation-curable and thermally curable binders known from DE-A 26 31 949, containing blocked polyisocyanates. DE-A 26 31 949 describes radiation-curable binders which comprise a blocked polyisocyanate, in the form of oxime, malonate, acetoacetate, caprolactone, phenol, etc. The coating films that have formed receive a thermal aftertreatment. The manner of thermal crosslinking described therein, which involves the reelimination of isocyanates, is not always unproblematic, however, with regard to the elimination products that form (e.g., pyrazoles, triazoles), some of which remain in the film and act, for example, as plasticizers. Consequently, there continues to be a need for alternative crosslinking mechanisms.